1. Field of the Invention
This invention relates to a riser reactor and a process for catalytic cracking of a hydrocarbonaceous feed, and more particularly a process for cracking a hydrocarbonaceous feed to gasoline range hydrocarbons.
2. Description of the Related Art
Fluidized catalytic conversion processes, such as fluidized catalytic cracking for the processing of petroleum fractions are well known. In a fluidized catalytic cracking process, a hydrocarbon oil feedstock is contacted with a catalyst in a reaction zone under conditions such that the hydrocarbon feedstock is converted into desired products accompanied by the deposition of coke on the surface of the catalyst particles. Such systems may comprise a transport or riser type reaction zone through which the feed hydrocarbon and a solid particulate catalyst suspended in feed hydrocarbon are passed concurrently. The reaction products and catalyst are discharged from the riser reaction zone into a separation zone in which hydrocarbons and normally gaseous by-products of the cracking reaction are separated from the catalyst.
In a riser reactor, as the mixture of catalyst and hydrocarbon feed passes upwardly through the reaction zone, the catalyst becomes partially deactivated due to the deposition of coke thereon and is referred to as "spent" catalyst as contrasted with regenerated or "fresh" catalyst. The spent catalyst from the reaction zone may be regenerated by reaction with oxygen or air.
In an ideal riser reactor the oil and catalyst move in concurrent upflow at relatively high velocities (50-100 ft/sec). At these velocities both the oil and catalyst move in plug flow without any slip. The fresh catalyst is contacted with the fresh feed and the spent catalyst is contacted with the products. As a result of this and the short residence time of the catalyst in the reactor, the extent of the secondary cracking reactions, which destroy gasoline range components to form coke and gas, is minimized. This is a very important characteristic of riser reactors. There is a characteristic rapid decline of catalyst activity in riser reactors due to the coke deposition on the catalyst. As a result of this, the bulk of the conversion occurs at the very bottom of the riser and oil/catalyst contacting at the inlet is very important.
In designing a pilot plant size riser reactor for use in testing new catalysts it is important to match commercial riser reactor performance. To do this it is necessary to design the reactor such that: (1) the catalyst and oil are contacted in a concurrent upflow or down flow fashion so that the feed contacts fresh catalyst and the products contact spent catalyst; (2) the reactor is operated at steady state with oil and catalyst fed and withdrawn continuously so that activity and coke profiles can be maintained along the length of the reactor; and (3) the catalyst feed rate is relatively low in order to minimize the amounts of oil and catalyst needed for each evaluation and to speed up generation of data.
A design that meets these requirements is a small upflow or down flow riser. However, such a reactor is very difficult to build and operate because it is extremely difficult to continuously feed a small flow of catalyst and to measure it in a reproducible manner. Additionally, it is difficult to maintain a constant catalyst to oil ratio in a riser with very small flows. Also, if the diameter of the reactor is too small it will be extremely difficult to circulate catalyst without encountering plugging problems and severe wall effects and to obtain effective catalyst and oil contacting at the inlet.
A number of different designs for riser reactors or fluidized bed reactors have been proposed. U.S. Pat. No. 4,295,967 (filed May 23, 1980) discloses an apparatus comprised of multiple fluidized beds provided with one or more trays having a number of inverted frustum-shaped depressions with an opening located therein. U.S. Pat. No. 4,725,408 (filed Feb. 16, 1988) discloses an apparatus for catalytic cracking of a selected portion of a hydrocarbon feedstock compri sing a riser reactor and a catalyst regenerator, a regenerated catalyst cooler, and an absorber. German Democratic Republic Patent No. 145-888 discloses a main fluidized reaction bed supplied with solids entrained by gas rising vertically From a secondary fluidized bed or other reaction system. The flowing medium containing the powdered solids passes through a diffuser grid separating the two beds and having apertures with a lower converging section and upper diverging section, separated by a diffuser throat.
It has now been discovered that a particular riser reactor design produces improved results in catalytic cracking.